Sintered ceramic products as mass-produced products complicated in shape are industrially produced by molding such powders as alumina, zirconia, silicon carbide and silicon nitride as raw materials into desired shapes according to injection molding or slip casting, followed by dewaxing, and then igniting the thus-obtained powder molded products at a temperature required for sintering.
The injection molding referred to above is a molding method in which a binder exhibiting plasticity as a whole and making molding easier such as, for example, polystyrene, polyethylene, diethylene phthalate, paraffin, fatty acid ester, or polyvinyl alcohol, is added to and kneaded with, for example, alumina powder as mentioned above in an amount of 20 to 35 parts by weight based on 100 parts by weight of the powder, and the kneaded mixture is charged under pressure into a desired shape of a mold and molded. The resulting powder-molded product is taken out of the mold and the binder is vaporized and removed by heating, followed by igniting to obtain a sintered ceramic product of a desired shape.
The slip casting referred to above is a casting method in which 20 to 40 parts by weight of a binder which is water or a mixture of water with an alcohol, as well as a small amount of a peptizer such as HCl, AlCl.sub.3, NaOH or water glass, are added to and thoroughly mixed with 100 parts by weight of, for example, alumina powder as mentioned above to obtain a stable slip having fluidity and difficult for the powder to precipitate, then this slip is poured into a mold of a porous material such as gypsum, allowing at least the binder contained in the slip to be absorbed into the mold until the slip has no longer fluidity, and then the resulting powder-molded product is taken out from the mold. The powder-molded product thus obtained still contains the binder, e.g. water, usually in an amount of 10% to 15% by weight. Therefore, as in the case of injection molding, such remaining binder is vaporized and removed by heating, followed by igniting to a temperature of, say, 1,300.degree.-2,300.degree. C., whereby a sintered ceramic product can be obtained.
A thermoplasticizer, a plasticizer, a dispersant and a solvent added to a powder in the injection molding and slip casting in the present invention will hereinafter be named generically as "binder". And the operation for vaporizing and removing, by heating or any other suitable means, the binder remaining in a molded product obtained according to the foregoing injection molding or slip casting, will hereinafter be referred to as "dewaxing" which term is commonly used by those skilled in art.
However, sintered ceramic products resulting from dewaxing and subsequent igniting of powder molded products obtained by the foregoing method, i.e., injection molding or slip casting, involve the problem that they are often defective (incapable of being used as products) due to cracking or delamination.
Further, once such defects occur in the interior of the sintered ceramic products, it is difficult to find out the defects at the stage of commercialization, so parts of such defective products are commercialized as they are, thus causing breakage in use. This is a serious problem.
In this connection, it is to be specially noted that such defects as cracking and delamination occur in the dewaxing step in most cases.
More particularly, if a binder remains in a powder-molded product, the remaining binder will vaporize rapidly when igniting the powder-molded product into a sintered ceramic product, thus causing fracture or cracking in the same product. To prevent this, that is, to remove the binder, the powder-molded product is subjected to dewaxing before the ignition. Inevitably, therefore, it is desirable to remove the binder as completely as possible in the dewaxing step.
As previously noted, however, powder molded products contain not less than 10 wt. % of a binder even in the case of slip casting and a larger amount, not less than 20 wt. %, of a binder in the case of injection molding. It is essentially extremely difficult to vaporize and remove the binder by heating from the powder molded products containing the binder in such a large amount without causing fracture or cracking because a large expansion force of the binder induced by the heat vaporization is exerted strongly on the powder molded products whose mechanical strength is very low.
Therefore, this process has heretofore been carried out by heating the powder molded products to a temperature of 600.degree. C. or so at the highest at atmospheric pressure or under a pressure of 5 kg/cm.sup.2 or lower to remove the binder through evaporation, efflux or combustion. In order to keep low the expansion force of the binder, there is adopted an extremely low heat-up rate for the powder molded product which ranges from 1.degree. to 3.degree. C./h. The dewaxing step usually requires a long period of 5 to 7 days because it cannot help adopting such low heat-up rate, thus impeding the productivity markedly.
In injection molding, moreover, a large amount of the binder used would result in increased volume of voids formed after removal of the binder in the dewaxing step. Therefore, it is required that the amount of the binder used be as small as possible and that the binder be easily molded and have properties capable of being easily heat-vaporized. However, there has been the problem that it is essentially extremely difficult to satisfy all of the above requirements even in the use of such expensive materials as the foregoing polystyrene and polyethylene.
Further since the powder molded products after dewaxing are almost zero in mechanical strength, fracture or cracking is apt to occur in the course of transfer to the next sintering step. In order to avoid this, it has been necessary to minimize vibration and deflection.
Thus, although the productivity is extremely low and the production is performed through extremely careful operations, not a few sintered ceramic products obtained are defective. This has mainly been attributable to the defects of powder molded products occurring in the dewaxing step.
Having made extensive studies for thoroughly eliminating the drawbacks of the prior art, the present inventors found out a method capable of suppressing the occurrence of such defects as fracture and cracking to a remarkable extent as compared with that in the prior art even when the binder was removed in an extremely short time. In this way the present invention was completed.